Field of the Invention
The present invention relates to an optical glass, a preform, and an optical element.
Related Art
Optical systems such as digital cameras and video cameras and the like, to varying degrees have blurring referred to as aberrations. These aberrations are classified as monochromatic aberrations and chromatic aberrations, and in particular, chromatic aberrations are strongly dependent on the material characteristics of the lenses used in the optical system.
Generally, chromatic aberrations are corrected by combining low dispersion convex lenses with high dispersion concave lenses, but these combinations can only correct aberrations in the red region and green region, while aberrations in the blue region remain. These aberrations in the blue region which cannot be eliminated are referred to as secondary spectra. In order to correct these secondary spectra, it is necessary to carry out optical design which takes into account shifts in the g-line (435.835 nm) of the blue region. At this time, as an indicator of the optical characteristics which are noted during the optical design, the partial dispersion ratio (θg,F) is used. In the above described optical system combining low dispersion lenses and high dispersion lenses, by using an optical material having a large partial dispersion ratio (θg,F) at the low dispersion lens side, and an optical material having a small partial dispersion ratio (θg,F) at the high dispersion lens side, secondary spectra can be well corrected.
The partial dispersion ratio (θg,F) is shown by the below formula (1).θg,F=(ng−nF)/(nF−nc)  (1)
In optical glasses, there is an approximately linear relationship between the partial dispersion ratio (θg,F) which shows the partial dispersion properties in the short wavelength range, and the Abbe number (νd). The straight line which shows this relationship, shown by a straight line connecting two points when plotting the partial dispersion ratio and the Abbe number of NSL7 and PBM2 in a Cartesian coordinate system where the partial dispersion ratio (θg,F) is the ordinate and the Abbe number (νd) is the abscissa, is referred to as the normal line (refer to FIG. 1). Normal glasses which are the standards of the normal line differ among different optical glass manufacturers, but this is defined by each manufacturer according to approximately equal slopes and intercepts. (NSL7 and PBM2 are optical glasses manufactured by Ohara, Inc., and PBM2 has an Abbe number (νd) of 36.3 and a partial dispersion ratio (θg,F) of 0.5828, and NSL7 has an Abbe number (νd) of 60.5 and a partial dispersion ratio (θg,F) of 0.5436.
Herein, as glasses having an Abbe number (νd) of 30 to 42, for example, the optical glasses according to Patent Documents 1 and 2 are known.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2002-029777
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-239478